Optical signals are becoming increasingly prevalent in transmitting information signals such as audio, video and data signals. Accordingly, there is an increasing need to characterize such optical signals. Conventionally, an optical signal has been characterized by converting it to an electrical signal, and characterizing the electrical signal as a proxy for the original optical signal. However, the conversion process introduces errors and uncertainty in the characterization of the optical signal.
As an alternative to the electrical conversion process described above, it has been proposed to characterize an optical signal by sampling the optical signal in the optical domain. An example of an optical domain optical signal sampling device is disclosed in Japanese Patent Publication H6-63869B, in which optical sampling pulses and the optical signal to be characterized, which will be called the signal-under-test, are subject to wavelength mixing using a non-linear optical crystal. Specifically, optical domain sampling is performed by controlling the polarization direction of the signal-under-test, spatially overlapping the signal-under-test and the optical sampling pulses and passing the overlapped signal through a non-linear optical material. The resulting optical samples pass from the non-linear optical material to a photodetector. The waveform of the signal-under-test is then determined from the electrical signal generated by the photodetector.
The optical domain optical signal sampling device just described has a very low conversion efficiency because of its use of a non-linear optical crystal. The shortcomings of a very low conversion efficiency can be overcome, to some extent, by using optical sampling pulses having a very high intensity. However, light sources capable of generating short-duration optical pulses with a sufficient intensity are not readily available at an economic price. Accordingly, the optical domain optical signal sampling device just described does not lend itself to practical applications.
Accordingly, what is needed is an optical domain optical signal sampling device that provides a high conversion efficiency and that is practical to manufacture.
The invention provides an optical domain optical signal sampling device that comprises an electrical sampling pulse source and an electrically-controlled optical modulator. The electrically-controlled optical modulator comprises electro-optical material, an optical waveguide located in the electro-optical material and including a bifurcated region, and electrodes disposed along the bifurcated region. The optical waveguide is arranged to receive an optical signal-under-test. At least one of the electrodes is connected to receive electrical sampling pulses from the electrical sampling pulse source. The electrical sampling pulses generate an electric field between the electrodes that differentially changes the refractive index of the electro-optical material in the bifurcated region of the optical waveguide to sample the optical signal-under-test.
The electrical sampling pulse source may include a photoconductive switch having an output connected to at least one of the electrodes of the electrically-controlled optical modulator. The photoconductive switch is operable to generate the electrical sampling pulses.
The optical domain optical sampling device may additionally comprise a light source operable to generate optical pulses and arranged to illuminate the photoconductive switch with the optical pulses to cause the photoconductive switch to generate the electrical sampling pulses.
The electrically-controlled optical modulator may be a first electrically-controlled optical modulator, and the optical domain optical signal sampling device may additionally comprise a DC bias supply and a second electrically-controlled optical modulator arranged in tandem with the first electrically-controlled optical modulator. The DC bias supply is connected to set the first electrically-controlled optical modulator and the second electrically-controlled optical modulator to opposite states. The electrical sampling pulse source is structured to provide first electrical sampling pulses to the first electrically-controlled optical modulator and second electrical sampling pulses, delayed relative to the first electrical sampling pulses, to the second electrically-controlled optical modulator. The electrical sampling pulses momentarily reverse the states of the first electrically-controlled optical modulator and the second electrically-controlled optical modulator.
The second electrical sampling pulses are delayed relative to the first electrical sampling pulses by less than the pulse width of the electrical sampling pulses.
The optical domain optical signal sampling device may additionally comprise a photodetector coupled to the optical pulse output of the optical waveguide. The photodetector may include a first electrical output, and the optical domain optical signal sampling device may additionally comprise an optical tap, a correction signal generator that includes a serial arrangement of an auxiliary photodetector and a controlled attenuator, and a differential amplifier. The optical tap includes an input arranged to receive the optical signal-under-test, a secondary output, and a main output optically coupled to the electrically-controlled optical modulator. The correction signal generator is optically coupled to the secondary output of the optical tap and includes a second electrical output. The differential amplifier includes inputs electrically connected to the first and second electrical outputs, respectively, and an output that provides electrical samples of the optical signal-under-test and that is additionally connected to the control input of the correction signal generator. The attenuator may be an electrical attenuator or an optical attenuator.
The optical domain optical signal sampling device according to the invention provides a high conversion efficiency and is relatively easy to manufacture.